test: cover jobs in Settings, and unpick two shared selectors

The spec worth having is not that the tab is gone -- that is one line
of a table -- but that nothing became unreachable when it went. #24
promises that no action is ever unreachable at any supported size, and
deleting a destination is exactly the change that quietly breaks it.

Two existing selectors had to give. `config-section .header` is
ambiguous the moment a section holds a job, because `job-details-drawer`
carries that class too -- so `settings-reach.spec.ts` locates a
disclosure by role and name instead. And `page-header`'s and
`offline-icons`'s view lists lose an entry each.

Closes #27

frontend/test/components/keyboard-reach.test.ts

@@ -49,7 +49,7 @@ describe('<app-sidebar> is reachable', () => {
 
     const items = shadowAll(el, 'li button');
 
-    expect(items).toHaveLength(11);
+    expect(items).toHaveLength(10);
     expect(items.every((item) => item.tagName === 'BUTTON')).toBe(true);
   });